The invention relates to a method of melting together the axial ends of bunched fibers of thermoplastic material, wherein the fiber ends are brought into contact with a heated surface of a stamp. The invention relates further to a device for attaching tufts of bristles for use in brushes to carrier plates of thermoplastic material. The carrier plates with the tufts of bristles attached thereto are incorporated in brush bodies, in particular for the fabrication of tooth brushes.
Several methods are known for the fabrication of brushes. In principle, brush bodies, having an array of holes corresponding to the desired array of bristles, can be provided. The tufts of bristles are then inserted into the holes of the brush body and anchored therein. The anchorage of the tufts of bristles in the brush body by means of anchor platelets or loops requires, however, highly performant and hence expensive machines.
According to an alternative fabrication method for brushes, the tufts of bristles are attached to a carrier plate that then is built into a brush body. The carrier plate can be joined to the brush body by injection-moulding around it or by welding. The carrier plate will be provided with holes according to the desired hole pattern, the utilization ends of the tufts of bristles projecting out of one surface of the carrier plate, and the axial ends of the tufts of bristles to be anchored in the brush protruding slightly out of the opposite side. A heated stamp is pressed against those ends of the tufts of bristles that are to be anchored in the brush body, melting together the ends of the tufts of bristles and possibly deforming them into knobs. During the subsequent separation of the stamp from the melted fiber ends, sticky threads and smearing of the viscous melted synthetic material may occur. Since, furthermore, the ends of the bristles as well as the carrier plate are heated, it is difficult on the one hand to effect the deformation of the bristles necessary for a perfect anchoring, and to prevent on the other hand an unwanted deformation of the carrier plate, all the more since the carrier plate and the bristles usually are made of different synthetic materials.
The invention provides a method of melting together the axial ends of bunched fibers of thermoplastic material, wherein the fiber ends are brought into contact with the heated surface of a stamp. According to the invention, the body of the stamp is heated by passing a controlled electric current through it, enabling extremely rapid and precisely controllable temperature changes of the stamp.
In a first variant of the invention, the fiber ends are brought into contact with a heated surface of a stamp, which then is cooled abruptly. Only after cooling of the surface has occured, the fiber ends are separated from it. In this way, the melted fiber ends can be removed cleanly from the heated surface and show an overall shape that is determined by the geometry of the surface. In this variant the application of a non-stick coating is advantageous.
Like in the first variant, in a second variant according to the invention the fiber ends are first brought into contact with a surface heated to a first temperature. The surface is then separated from the fiber ends while maintaining, however, the temperature of the surface. After that, the surface is heated up to a second, higher temperature in order to vaporize any remainder of the fiber material adhering to the surface. In a final step according to the method, the surface is cooled again to the first temperature. In this variant, the adherence properties of the heated surface with respect to the heated fiber material are uncritical, a non-stick coating being hence unnecessary.
Both variants of the invention are especially suited for the fabrication of arrays of bristles to fabricate brushes. Fibers for the fabrication of brushes mostly consist of a thermoplastic material like polyamide (xe2x80x9cnylonxe2x80x9d). This material can be deformed easily with the inventive method.
The invention further provides a device for attaching tufts of bristles to carrier plates in order to manufacture brushes, enabling a controllable and well reproducible operation of the stamp upon the ends of the bristles, assuring the desired deformation of the ends of the bristles without any unwanted deformation of the carrier plate. In the device according to the invention, the stamp is heated by an electric current and can be cooled by a flowing cooling agent. The stamp can be heated rapidly and in a specific way by an electric current, especially if, according to the preferred embodiment, it has a low heat capacity, so that it quickly can be cycled through different temperature phases, including cooling by the cooling agent. Since the ends of the bristles are heated only a very short time and instantanously cooled again afterwards, a smearing of the heated bristle material on the carrier plate is avoided. By the same token, the stamp may alternatively be heated to a second, higher temperature after having been withdrawn from the fiber ends in order to vaporize any remainder of the fiber material adhering to the surface. The carrier plate itself is warmed up only slightly since the stamp is heated only for a short time to the temperature needed to melt together the ends of the bristles, and is removed or cooled instantaneously thereafter. Controlling the electric current, particularly via pulse width modulation, allows a good control of the intensity and the duration of the heating process.
Preferably, the stamp comprises a body of electrically conducting material, on which two electrical high-current terminals in the shape of bent-off contact shoes are formed. The body of the stamp has a thin-walled stamp plate that may be strengthened by an angled bordering strip. Suitable materials for the manufacturing of the stamp are metals, having on the one hand sufficent mechanical strength in order to assure the desired low heat capacity needed for a fast change of temperature, and showing on the other hand only a moderate resistivity, so that only an uncritical electric voltage is needed to achieve the electrical heating power. Although, in this case, the required heating currents have values of some hundred Amperes and more, for example 200 Amperes at a voltage of 7 V, such high currents can well be controlled using available semiconductor components. In view of these criteria, stainless steel, titanium and NiCr-containing alloys are suitable materials for the fabrication of the stamp.
In order to cool the stamp, compressed air is preferably used. Due to the low heat capacity of the stamp, only a short time is needed to cool it down by directing compressed air against it, so that cycle times of about one second are feasable.
In the preferred embodiment of the device, a stamp carrier plate is provided with a plurality of stamps forming a group, and the same number of carrier plates is inserted into the corresponding openings of a supporting plate opposite the stamps. Preferably, the stamps are electrically connected in series at the stamp carrier plate, so that the intensity of the heating current does not increase. This measure is expedient especially if the stamp carrier plate together with the stamps is reciprocated with respect to the carrier plates incorporated in the supporting plate, in which case the electrical leads for the heating current have to be moved accordingly. As a consequence, large conductor cross-sections would be disadvantageous.